1. Field of the Invention
The present invention generally relates to a sheet feeding apparatus equipped in a copier, facsimile, image-scanner, or similar equipment, for feeding original documents or recording sheets one by one.
2. Description of the Background Art
FIG. 1 is a cross-sectional view of a portion of a sheet feeding apparatus of a background art device which includes a separating roller 3. FIG. 2(a) is a perspective view of a supporting mechanism of the separating roller 3 of FIG. 1, and FIG. 2(b) is an exploded view of the separating roller 3. In FIG. 1, the sheet feeding apparatus includes a feeding tray 1 for holding a stack of recording sheets PP as a sheet locating portion, a press lever (not shown) for pressing a top of the stack of recording sheets PP toward the tray 1, arranged above the tray 1, and a stopper (not shown) for locating the sheets PP at a predetermined loading position on the tray 1.
In addition, the sheet feeding apparatus includes a pick-up roller 2 contacting the top of the stack of sheets PP to pick-up a topmost sheet P. The pick-up roller 2 may be formed of, for example, a circular or non-circular shaped roller, an eccentric roller, a non-circular eccentric roller, a circular or non-circular shaped roller with a frictional member wound along a rim, etc. The type of pick-up roller is chosen in relation to a structure of the loading portion. As another type of loading portion, a sheet feeding cassette is disclosed in Japanese Laid-Open Utility Model Application No. 06-87348.
Furthermore, the sheet feeding apparatus of FIG. 1 includes a separating roller 3 arranged at a lower reach of the pick-up roller 2 and a feeding roller 10 arranged at an opposite position to the separating roller 3. The feeding roller 10 is rotated by a feeding torque in a sheet feeding direction.
Next, referring to FIG. 2(a), a mechanism for driving the separating roller 3 is described. The driving mechanism includes a driving shaft 4, a driving gear 5, a follow gear 8, and a torque limiter 9. These members act to transmit a rotational power to the separating roller 3 to rotate separating roller 3 in a reverse direction against a sheet feeding direction. Further, a bracket 6 is movably supported by the driving shaft 9, and supports the driving gear 5, the follow gear 8, the torque limiter 9, and the separating roller 3 using a follow shaft 7 as a center axis. A spring 11 pushes the bracket 6 toward the feeding roller 10, see FIG. 1.
Next, referring to FIGS. 2(b) and 3, a torque mechanism is described. As shown in FIG. 2(b) and FIG. 3, the following shaft 7 has a pin 7a, and the pin 7a is inserted into a gap of the follow gear 8 to mesh with the driving gear 5, for fixing each other. As shown in FIG. 3, the torque limiter 9 includes a follow part 9b and a driving part 9a, and when a torque greater than a predetermined upper limit acts between follow part 9b and driving part 9a, the follow part 9b slips against the driving part 9a. On the other hand, the driving part 9a is connected with the pin 7a and, therefore, the driving part 9a rotates with the shaft 7 integrally on all occasions. The separating roller 3 is connected and fixed with the follow part 9b in relation to a rotating direction. Therefore, when an acting torque between the follow part 9b and the driving part 9a is less than the upper limit, the separating roller 3, and the follow part 9b as an axis of the separating roller 3, integrally rotate with the driving part 9a, the follow shaft 7, and follow gear 8. On the other hand, when an acting torque is greater than the upper limit, the separating roller 3 and the follow part 9b can rotate independently.
Next, referring to FIGS. 1 to 4(a) and 4(b), a separating movement of a background sheet feeding apparatus is described.
To the begin with, stack of sheets PP is set on the feeding tray 1 and a tip of sheets PP is pushed on a stopper (not shown) so as to locate the sheets PP at a predetermined loading position. As the sheets PP are fed, a lever (not shown) presses and gives the sheets PP on the tray 1 a prefixed moving friction, and the stopper leaves from the tip of the sheets PP. Then, a motor (not shown) transmits a torque to the pick-up roller 2 via a clutch mechanism (not shown), which rotates the pick-up roller 2 exactly by one turn each one action. Thereafter, as shown in FIG. 1, the pick-up roller 2 rotates to feed a topmost sheet P of the stack of sheets PP, and at an infant phase, the pick-up roller 2 stops contacting a surface of the fed sheet P. In other words, the pick-up roller 2 contacts a surface of the topmost sheet P for only a part of a phase-section. In this operation, a power for feeding the topmost sheet P acts directly on the topmost sheet P. However, an indirect feeding force also acts on a lower second sheet or further sheets under the topmost sheet P by friction occurring between the topmost sheet P and the lower second and further sheets. As a result, one or some further sheet(s) below the topmost sheet P may be (are) pushed out and inserted to a nip portion between the feeding roller 10 and the separating roller 3. That is, an improper feeding of several sheets, rather than just the topmost sheet P, can occur.
A reverse torque transmitted from the driving shaft 4 causes the separating roller 3 to rotate in an opposite direction (e.g., counterclockwise) of the feeding direction via the torque limiter 9. On the other hand, a feeding torque transmitted from a motor causes the separating roller 3 to rotate in a feeding direction (e.g., clockwise) via the feeding roller 10.
When there is none or only one paper sheet between the separating roller 3 and the feeding roller 10, as shown in FIG. 4(a), since the feeding torque is relatively larger than the reverse torque, the separating roller 3 is rotated in a feeding direction subordinately by the feeding torque. That is to say, in the torque limiter 9, the follow part 9b is slipped against the driving part 9a so that the separating roller 3 rotates in a feeding direction (e.g., clockwise) as shown in FIG. 4(a).
On the other hand, when two or more sheets of paper P,PPP are at the nip between separating roller 3 and feeding roller 10, or when the separating roller 3 does not contact the feeding roller 10 altogether, as shown in FIG. 4(b) the feeding torque is relatively smaller than the reverse torque, and as a result, the separating roller 3 is rotated in a reverse direction (e.g., counterclockwise) against the feeding direction by the reverse torque. This results because friction occurring between the topmost sheet P and a second sheet PPP, which is smaller than a friction occurring between the top sheet P and the feeding roller 10, reduces a torque transmitted from the feeding roller 10 to the separating roller 3. Therefore, in the torque limiter 9, the follow part 9b rotates in a same direction with the driving part 9a.
Consequently, by this action, the sheet PPP is pushed back toward the feeding tray 1, and only the topmost sheet P is fed in the feeding direction.
In a sheet feeding and separating action mentioned above, in the first place, as shown in FIG. 4(a), a topmost sheet P is inserted and fed at the nip between the separating roller 3 and the feeding roller 10, and accordingly the separating roller 3 subordinately rotates in the feeding direction shown by F in FIG. 4(a). And then later, as shown in FIG. 4(b), the sheet PPP is inserted and fed by a frictional force between the sheet PPP and the sheet P. While there are at least two sheets between the nip, the separating roller 3 rotates in the reverse feeding direction shown by R in FIG. 4(b) and the separating roller 3 pushes back the sheet PPP to ensure proper feeding of only the topmost sheet P.
However, a drawback with this operation is that after the separating roller 3 pushes back the sheet PPP, the separating roller 3 starts to rotate in the feeding direction again, and the sheet PPP may then be fed in the feeding direction again during a conveyance of the sheet P. The separating roller 3, therefore, switches repeatedly between rotating in the feeding direction (e.g., clockwise) and the reverse feeding direction (e.g., counterclockwise) during conveyance of the sheet P. A torque reaction of this repeated switching action issues a shock and a vibration to the separating roller 3, and the shock and the vibration are propagated to the feeding roller 10 or the follow shaft 7, or the driving shaft 4, or the bracket 6, etc., and this results in mechanical noise. Further, a frequency of the vibration and a level of the noise increase with an increasing in a rotational speed of the separating roller 3. Especially, when the vibration-frequency approaches a resonance frequency region of the follow shaft 7, or bracket 6, etc., the noise is amplified by the resonance phenomenon. The shock, moreover, is increased with shortening of a period of the repeated action. In the other words, as a transitional period from a slipping condition to a following condition, or from a following to a slipping of the follow part 9b in regard to the driving part 9a, is shortened, a level of the shock increases.
Accordingly, it may become necessary to decrease the noise or the shock by making parts around the separating roller 3, such as the follow shaft 7, bracket 6, etc., of high stiffness-materials, or the torque limiter 9 may have a characteristic prolonging of the transitional period.
However, these measures have problems such that the sheet feeding apparatus then becomes expensive or complex.